The invention describes a measurement system, in particular for a water phantom, with at least one movement axis on which at least one holder for a radiation detector is arranged, wherein the holder can be moved along at least one movement axis, wherein a control unit is present, which accepts and executes commands for controlling the movement axis.
By way of example, such a measurement system is used in a water phantom in radiation therapy within the scope of quality control or commissioning of irradiation devices, such as accelerators. Here, the radiation field of the irradiation device is measured with the aid of a radiation detector, which is for example moved in a water container which is filled with water (a so-called water phantom).
The water in the water phantom has a specific absorption rate for radiation which approximately corresponds to that of human tissue. So that, for example, a measurement of the two-dimensional distribution is meaningful, it is decisive that the absorption is the same at all measurement points of the measurement plane. This is achieved by virtue of the water level above the detector always remaining constant.
In the process, the coordinate system in which the radiation detector is moved is generally defined by the mechanical movement axes, along which the movement takes place in one, two or three spatial dimensions. The mechanical movement axes must therefore be aligned precisely with respect to the water surface of the water phantom so that the water level is exactly equal at various positions of a measurement plane.
Adjusting the movement axes by hand or using an electric motor is the current prior art. In one embodiment, the movement axes are fixedly assembled on the water container. Now the whole unit, including water container and movement axes, is aligned horizontally and therefore parallel to the water surface. In another embodiment, the movement axes are attached on the water container in adjustable fashion and can be horizontally aligned independently of the latter. Here, there is no need to align the water container as long as all that is demanded is a parallel alignment with the water surface, but not, at the same time, a parallel alignment with the walls of the water container.
After the alignment, a movement along the fixed movement axes is always parallel or perpendicular to the water surface.
However, this mechanical alignment of the movement device with respect to the water surface requires much care and is very time-consuming. Moreover, the movement device must have additional means for the alignment, as a result of which the device is more complicated and expensive.